1. Field of the Invention
This invention pertains to the field of hydraulic rotary pumps and to assemblies combining several such pumps.
2. Description of the Prior Art
An hydraulic pump includes an inlet port, which is supplied with fluid from a reservoir or other source of hydraulic fluid, an outlet port connected to a pressure line, pumping chambers to carry the fluid from inlet port to outlet port, and a mechanical drive for activating the pump chambers. In most hydraulic pumps, the pumping chambers increase in size as they rotate toward the inlet, thereby creating a partial vacuum in the chambers. The vacuum at the inlet creates a pressure difference so that fluid flows from the reservoir into the pumping chambers. The chambers then decrease in size as they rotate toward and approach the outlet in order to push fluid from the chambers into the hydraulic system.
There are many different types of pumps used to pressurize hydraulic systems. Such pumps are generally fixed, constant displacement, or adjustable displacement versions.
Cavitation is a condition that often occurs in a pump inlet and fluid is supplied at an insufficient rate to supply enough oil to keep the inlet filled. The condition produces bubbles in the hydraulic fluid, which implode as they are exposed to system pressure at the outlet of the pump. In addition to excessive pump speed, other reasons for cavitation include excess restriction in the inlet line, too low a location of the reservoir fluid below the inlet, and high viscosity of hydraulic fluid. Cavitation causes erosion of metal within the pump and increases deterioration of hydraulic fluid. A badly cavitated pump makes a very distinctive noise as the bubbles implode under pressure. One way to avoid cavitation in a pump is to maintain the pump reservoir filled with fluid to a level sufficient to fill the pump inlet.
In an automotive vehicle equipped with an engine and an automatic transmission that are arranged with their axes disposed laterally and located between the drive wheels of the vehicle, hydraulic fluid located in the sump moves under the influence of centrifugal force from side to side as the vehicle maneuvers and turns. This action causes hydraulic fluid to be removed from the pump inlet and may cause pump cavitation unless corrective action is taken.
In an automatic transmission, fluid can be drawn from the transmission sump, a receptacle located at the lowermost extremity of the transmission and moved to a reservoir located adjacent the pump inlet. To accomplish this, a scavenge pump is used to move fluid from the sump to the reservoir.
In an automatic transmission, various portions of the hydraulic control circuit have differing requirements with respect to the magnitude of pressure and flow rate of an hydraulic fluid. For example, the portion of the hydraulic circuit, through which an hydrokinetic torque converter, lubrication system, and fluid cooler are supplied, has a moderate low pressure requirement, whereas the portion of the circuit, through which a line pressure regulation and hydraulically actuated friction elements, friction clutches and brakes are supplied, has a high pressure and low flow rate requirement.
The rotary pump assembly disclosed in U.K. Patent 1,234,889 includes a Gerotor hydraulic pump having a rotor with radially directed gear teeth intermeshing with a gear wheel. Fluid is pumped from an inlet to an outlet in the space between the geared rotors.